Tuesday, October 15, 2013

I have been thinking a lot about Vitamin D lately. Wrapping
up field work at high altitude, coupled with my love of outdoor running means I
have spent a near fortune on sunscreen as of late.
We also had a baby with early stage jaundice in our study. His parents were
understandably concerned, and treated the jaundice with lots of breastfeeding
(see ABM treatment protocol here: http://www.ncbi.nlm.nih.gov/pubmed/20387269 )
and sunshine. In fact, he was put in his bassinet outside under a mosquito net
every time the monsoon eased up.

It was a stark contrast to my skin cancer concerns, where
twice daily I coated myself in any number of sunblocking chemicals. Reading the
labels, the products were safe for infants older than six months . . . below
that, ask a physician.The general
recommendation is to keep infants out of the sun and reduce the risk of sunburn
and UV exposure. The source of Vitamin D for infants is breast milk (or
formula). The Vitamin D in human milk comes from maternal synthesis.

Vitamin D synthesis by the body requires a UV wavelength of
290-300nm; this is only available when the UV index is above 3. The UV rays
absorbed by the skin convert the prohormone 7-dehydrocholesterol into
cholecalciferol. This travels via the bloodstream to the liver, where it is
metabolized into 25-hydroxyvitamin D. Synthesis is then almost done: the
hydroxyvitamin D travels to the kidneys, where it is converted to the
metabolically active dihydroxyvitamin D (Vitamin D). Vitamin D aids the body in
calcium absorption, and appears to play a major role in regulating insulin,
calcium, and phosphorus levels in the body.

And as most people know, skin color is directly associated
with UV absorption and Vitamin D production. Skin pigmentation is determined
largely by the amount of melanin – more melanin = darker skin. More melanin
results in increased UV deflection which means decreased risk of harmful UV
rays being absorbed (and a decreased risk of skin cancer) but increased risk of
Vitamin D deficiency at higher latitudes. UV light, and Vitamin D synthesis, is
thought to have played a major role in the evolution of skin color, with darker
skin colors found around the equator, where there is plenty of sunlight and
opportunity to make Vitamin D and UV damage is a bigger risk. Lighter skin
colors are found at higher latitudes as the amount of daily and direct sunlight
decreases: less melanin increases UV absorption (Antoniou et al., 2009). This
may be beneficial in preventing Vitamin D deficiency, including rickets. Some
populations, like Inuit, may also supplement through dietary sources of Vitamin
D (whale liver anyone?). Sunscreen is incredibly effective at blocking UV rays:
a SPF of 8 blocks 95% of the UV; SPF 15 99%.Other factors influencing vitamin D levels are body size,
specifically the amount of body fat individuals may have. Vitamin D is fat soluble. Extra Vitamin D is stored in fat cells, and may not be accessible
unless the fat is metabolized.

So how do you get enough Vitamin D without exposure to too
much sun? The good news is for most of us, especially during the summer, we get
enough in short bursts that our Vitamin D levels are pretty good. On a sunny
day, walking to and from your parking space at work or the grocery store or
similar is probably enough. The best estimates are 5-30 minutes of exposure,
from 10am to 3pm, 2-3 times a week are sufficient to meet most individual’s
Vitamin D needs, provided the face, arms, and neck are uncovered. Darker skin
tones will need more exposure. There is a handy online calculator where you can
put in your data (including latitude) and it will generate an estimate. The
human body is remarkably efficient at making Vitamin D: 10,000-20,000 IU can be
synthesized in 30 minutes.

However, nursing mothers will need more Vitamin D, as will
individuals with limited sun exposure, heavy use of sunscreen, darker skin
colors, living at higher latitudes (especially during the winter), higher body
fat, and vegetarians. Most milk sold in the United States is fortified with
Vitamin D, as are many breakfast foods. Between sunshine and food
fortification, most women are likely meeting their own needs.

But the real question you came for is about babies. Should
breastfed babies, especially exclusively breastfed babies, receive Vitamin D
supplementation? Or is supplementing mothers with extra Vitamin D an
alternative treatment strategy?

Vitamin D deficiency is probably fairly common: Choi et al., (2013)
reported a prevalence of 48.7% in Korean infants, with breastfed infants more
likely to bevitamin D
deficient than formula fed infants, likely reflecting fortification of infant
formula with supplemental vitamin D. Similarly high rates of Vitamin D
deficiency were reported in Turkish infants (Halicioglu et al., 2012). In the
United States, the incidence rate is approximately 25-40% for unsupplemented
exclusively breastfed infant. Infants need approximately 400 IU of Vitamin D
per day, and based on current estimates for human milk, infants are unlikely to
get sufficient Vitamin D from human milk alone.

“Despite the
association between sunlight exposure and human milk vitamin D concentration,
there are no reports of the effect of long-term sunlight exposure of the mother
on her milk vitamin D concentration.”Dawodu
A, Tsang RC. 2012 Adv Nutr3: 353-361.

However, we do have some evidence: a few studies do exist
looking at the relationship between maternal and milk Vitamin D levels, often
called antirachitic activity, as the measure includes both the biological
activity of Vitamin D and its metabolites. Most of these studies are
supplementation studies – providing mothers with additional vitamin D, rather
than relying on maternal synthesis.

One of the first major supplementation studies is that of Hollis
and Wagner (2004). Eighteen mothers at one month postpartum were enrolled into
one of two treatment groups: 1600 IU D2 + 400 IU D3 or 3600 IU D2 + 400 IU D3.
Mothers continued in the study for 3 months when milk antirachitic activity was
tested. Both groups showed an increase in milk antirachitic activity: group one
had a milk mean of 34.2 IU/L and group 2 a milk mean of 94.2 IU/L. However,
neither increase was sufficient to meet infant metabolic requirements.

This was followed by a study by Saadi et al., (2009). Working with a sample of Middle Eastern women,
Saadi et al., used two treatment groups: one receiving 2000 IU/day of Vitamin D
and the other receiving 60,000 IU/month. Mothers reported seven minutes per
week of sun exposure, low dietary intakes of Vitamin D rich fish, and had
undetectable antirachitic activity in their milk prior to entering the study. Supplementation
increased milk antirachitic levels in these women to 50 IU/L (10-63 IU/L),
within the range of US women relying only on incidental sun exposure for
synthesis. The 50 IU/L levels are considered low, and well below the
recommended intake for infants.

In a large meta-analaysis of available studies on Vitamin D
supplementation of mothers as a way of managing infant Vitamin D needs, Dawodu
and Tsang (2012) conclude that based on the evidence currently available, it is
unlikely that maternal supplementation could increase the antirachitic activity
of milk enough to meet infant requirements.

While human milk is almost always the ideal first food for
human infants, that does not mean it meets 100% of needs 100% of the time.
Specifically, given that human babies likely had plenty of sun exposure for the
majority of human evolutionary history (including as recently as our
grandparents and still in many parts of the world) there would have been
minimal selective pressure on increasing Vitamin D transfer into milk. Babies,
especially in tropical climates and during certain seasons of the year, may
have received plenty of sunlight, certainly enough for individual synthesis of
Vitamin D. Long term exposure to damaging UVs would have a byproduct, but
probably not as important as synthesizing enough Vitamin D to prevent rickets,
seizures, and other factors associated with low Vitamin D synthesis. Mothers
also, likely had plenty of exposure to sunlight, probably had much higher
levels of circulating Vitamin D, and greater amounts of it in milk. Vitamin D requirements were probably meet by
the mutual sun exposure of mothers and infants, and Vitamin D requirements
during infancy and childhood may have contributed to selection against melanin
at high latitudes and a reduction in skin pigmentation to maximize synthesis.

Figure 1: A mother and baby from Nurbi, Nepal. Babies are typically worn on the back or carried in baskets and receive plenty of daily sun exposure. Photo: Geoff Childs, used with permission.

In evolutionary medicine, we use the term mismatch to describe situations where
current behaviors have changed dramatically from similar behaviors throughout
human evolutionary history. That is not to suggest some sort of fictionalized
single environment that humans are perfectly adapted to, but a general
observation about how we likely cared for babies during most of our
evolutionary history and even today in many parts of the world, including my
field sites in the Philippines and the Himalayas.Babies and mothers were outside in the sun,
and had plenty of opportunities for Vitamin D synthesis . . . and also exposure
to harmful UV rates and sunburn. Further, with the continued degradation of the
ozone layer, the potential for sunburn and skin damage is high. And Vitamin D
supplementation of moms and babies is great solution.

Mismatch does not
have to mean pathology, and this is one of those great situations where
understanding why something isn’t present in milk can help us better understand
current clinical practice.